C. F. Keil and Franz Delitzsch on Genesis 38:27-28

Gen 38:27-28

Thamar brought forth twins; and a circumstance occurred at the birth, which does occasionally happen when the children lie in an abnormal position, and always impedes the delivery, and which was regarded in this instance as so significant that the names of the children were founded upon the fact. At the birth ויּתּן־יד “there was a hand,” i.e., a hand came out (יתּן as in Job 37:10; Pro 13:10), round which the midwife tied a scarlet thread, to mark this as the first-born.